A new, more comprehensive electronegativity scale

The electronegativity of atoms is one of the most well-known parameters for explaining why chemical reactions occur. Now, Martin Rahm, assistant professor of physical chemistry at Chalmers University of Technology (Gothenburg, Sweden; www.chalmers.se), has redefined the concept with a new, more comprehensive scale. His work, undertaken with colleagues Tao Zeng at Carleton University (Ottawa, Ont., Canada; www.carleton.ca) and Roald Hoffmann at Cornell University (Ithaca, N.Y.; www.cornell.edu), was published in a recent issue of J. Am. Chem. Soc.

Numerous electronegativity scales have been developed since the concept was first proposed by Swedish chemist Jöns Jacob Berzelius in the 19^th century, but most of these scales only cover parts of the periodic table, typically omitting various heavy or heaviest elements. The new scale covers elements 1 to 96 — the most comprehensive to date.

“The new definition is the average binding energy of the outermost and weakest-bound electrons — commonly known as the valence electrons,” explains Rahm. “We derived these values by combining experimental photoionization data with quantum mechanical calculations. By and large, most elements relate to each other in the same way as in earlier scales. But the new definition has also led to some interesting changes where atoms have switched places in the order of electronegativity. Additionally, for some elements, this is the first time their electronegativity has been calculated,” says Rahm.

For example, compared to earlier scales, manganese and zinc have both been moved in the ranking, relative to elements closest to them in the periodic table. Fluorine is still the most electronegative element in the new scale, but it is 1.3 eV less electronegative than helium, which helps to explain the non-existence of helium fluorides.

One challenge with electronegativity as a concept is that it is sometimes unable to predict chemical reactivity or the polarity of chemical bonds. A further advantage of the new definition is how it fits into a wider framework that can help explain what happens when chemical reactions are not controlled by electronegativity, says Rahm.